In recent computer systems, hard magnetic information recording disks, which are often called hard disks or hard drives, have become an integral element for data storage and retrieval. A hard disk comprises two principal components: a magnetic head and rigid magnetic disks (which are often referred to as "hard magnetic disks", or even "hard disks"). The magnetic head writes data onto the rigid magnetic disk through the change of the magnetic field in the magnetic head which is caused by the change of the electric current that flows therethrough in accordance with the output data from the central processing unit. On the other hand, the magnetic head also reads data from the rigid magnetic disk, which induces a voltage in the magnetic head in accordance with the data that have been magnetically stored on the magnetic disk. The voltage so induced is subsequently amplified and converted back into its original digital data form before it is transmitted to the central processing unit.
A rigid magnetic disk comprises a rigid substrate and a layer of magnetic recording medium coated on the surface thereof for magnetically recording information to be stored. To avoid damages that may be caused to the magnetic recording layer due to its frictional contact with the magnetic head, a protective lubricating layer is often coated on top of the magnetic recording layer.
Aluminum alloy is commonly used as the base material to make substrates for the rigid magnetic disks. In the manufacturing of rigid magnetic disks, the aluminum alloy is first cut into a desired shape and size, then fabricated to obtain the desired flatness, surface roughness, and surface texture. The later step is necessary in order to, for example, improve the stickiness between the aluminum substrate and the magnetic recording layer, improve the flying motion of the magnetic head, reduce the friction therebetween, etc. Then a layer of NiP coating often is applied, typically via an electroless plating procedure, to provide a desired hardness at the surface of the aluminum substrate. After the electroless NiP plating, the surface of the aluminum alloy substrate is then uniformly coated with a layer of a magnetic recording composition using a coating or sputtering method. Finally a protective layer is coated on the surface of the magnetic recording layer as discussed above.
The trend in the computer industry is to pursue products that are lighter, thinner, shorter, smaller, speedier and capable of providing higher performance. Consistent with this trend, metallic films such as Co--Cr and Co--Ni, which provide high saturation magnetic density, are becoming the mainstream material in providing the magnetic recording medium for hard disks. Use of these materials requires the use of substrates that have a higher degree of surface flatness and roughness. These materials also require a lower flying height of the magnetic head above the magnetic disk, in order to achieve the purpose of maximizing the density of the stored information. For the reasons that follow, aluminum substrates have severe limitations for use with these materials. First, because of its relatively inferior hardness, an electroless NiP plating must be applied to in order to achieve the required hardness. This complicates the fabricating process, and the flatness of the product made therefrom is still far from ideal. The aluminum substrate also lacks the requisite strength to make very thin substrates. These weaknesses limit the extent by which the size of aluminum substrate based hard disks can be reduced. In addition to the aforementioned problems, an aluminum substrate also suffers from the problem of having relatively high thermal expansion coefficient. This often results in a dimensional instability of the magnetic disk made from the aluminum alloy and causes problems during the tracking of the magnetic head to read data from the rigid magnetic disk.
An aluminum substrate has the advantage that, because it involves a relatively matured technology, the cost for making aluminum based rigid disks is lower. However, due to its severe limitations described above, a need exists in the computer industry to develop new materials that can substitute aluminum alloy to provide improved properties for use as substrates for computer hard disks of the next generation. Recently, it has been disclosed using ceramic, glass, or glass-ceramic to make substrates for computer hard disks. Ceramic materials are well-known to have high hardness and high strength; they also allow the production of substrates with improved surface flatness and roughness. Glass-ceramics are non-porous but have an inherently textured surface. They exhibit good surface hardness which is typically scratch resistant. Furthermore, glass-ceramic articles can be formed as glasses to the designed size and shape, and they often require only a minimum amount of finishing after sintering, thereby reducing the cost of processing.
These advantageous properties enable the substrates made with ceramic or glass materials to be smaller and thinner than their aluminum alloy counterparts. However, both glass and ceramic materials are non-conductors and they do not provide electromagnetic shield. As the hard magnetic disks become thinner and smaller and the data to be stored per unit area of disk space quantum-leaped by orders of magnitude, electromagnetic interference will inevitably become a serious problem for the glass or ceramic based disks and must be dealt with. Furthermore, the demand in substantially increasing the data transfer rate to and from the magnetic disk also creates a much more stringent requirement on the amount of electromagnetic interference that can be tolerated passing through the disk.
In U.S. Pat. No. 4,738,885, it is disclosed a substrate made by molding a powder mixture containing alumina powder, a sintering aid and an organic binder into a doughnut-like disk under a hydraulic press. The molded body was sintered at 1600.degree. C., followed by a hot isostatic pressure treatment at 1,500.degree. C. and 2,000 atmospheres pressure to form a substrate with reduced number of voids on the surface thereof. Finally, the substrate was polished with a diamond abrasive and further polished by lapping and fine polishing.
U.S. Pat. No. 4,833,001 discloses a glass substrate for a rigid magnetic disk having a finely and isotropically roughened surface which is obtained by applying a chemical etching treatment to the glass surface and optionally applying mechanical polishing treatment to the roughened surface. The glass substrates disclosed in the '001 patent often experience surface defects which affect its strength.
U.S. Pat. No. 4,971,932 discloses a substrates made from glass-ceramics by melting glass which is then formed into a glass substrate having desired shapes and geometries using conventional glass forming processes. The glass substrate is then heat-treated to obtain a crystallized phase to thereby enhance its strength. Surface texture with desirable roughness is obtained by controlled nucleation and crystallization of glass of suitable composition which produces a crystalline microstructure. Subsequent mechanical polishing and/or chemical etching is applied to obtain the desired surface roughness.
Japanese Pat. Pub. No. Sho-61-10052 discloses a substrate made from a method similar to that disclosed in the '885 patent, except that it utilizes zirconium oxide and the sintering and hot isostatic pressure treatment are conducted at a different set of conditions. Zirconium oxide is substantially more expensive than aluminum oxide.
Glass and/or ceramic substrates are also disclosed in U.S. Pat. Nos. 4,959,255, 5,087,481, 5,080,948. All of the above-mentioned non-conductor substrates, among other things, share a common problem in that none of them provides the capability to shield interference from electromagnetic waves. In an attempt to deal with this problem, Japanese Pat. No. 2214018 discloses a ceramic substrate which applies a coating of NiP between the ceramic layer and the magnetic recording layer to avoid electromagnetic interference. Japanese Pat. No. 1192015 discloses a similar substrate except that a superconducting material is utilized in place of the NiP plating. The principle of using a superconducting material is to utilize its dia-magnetic property to interrupt the penetration of magnetic field through the magnetic disk. In both patents, the interference problem was handled using a surface treatment that was applied after the substrate has already been made, i.e., after the ceramic green body has been densified. The NiP plating on the ceramic substrate defeats its inherent textured microstructure and the stickiness between the NiP layer and the ceramic substrate, after it has been sintered, is always a concern. Furthermore, the superconducting material requires a working condition below a liquid nitrogen temperature, thus necessarily complicates the problems of parts selection and drastically increases capital as well as operational costs. Other problems with the superconducting approach are also readily apparent, such as the low strength of the superconducting material, its lack of stability, low stickiness, etc. Also the sputtering of the magnetic layer would destroy the superconductivity, thus rendering only a very limited selection of suitable magnetic materials that can be used for successful implementation.